1. Field of the Invention
The present invention relates to an apparatus for manufacturing a semiconductor device having a circuit structured with a thin film transistor. For example, the invention relates to an apparatus for manufacturing an electro-optical device, typically a liquid crystal display device, and the structure of electric equipment mounted with such an electro-optical device as a component. Note that throughout this specification, the semiconductor device indicates general devices that may function by use of semiconductor characteristics, and that the above electro-optical device and electric equipment are categorized as the semiconductor device.
2. Description of the Related Art
In recent years, the technique of crystallizing and improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystallinity which is polycrystalline or microcrystalline, but is not a single crystal), in other words, a non-single crystal semiconductor film, formed on an insulating substrate such as a glass substrate by laser annealing, has been widely researched. Silicon film is often used as the above semiconductor film.
Comparing a glass substrate with a quartz substrate, which is often used conventionally, the glass substrate has advantages of low-cost and great workability, and can be easily formed into a large surface area substrate. This is why the above research is performed. Also, the reason for preferably using a laser for crystallization resides in that the melting point of a glass substrate is low. The laser is capable of imparting high energy only to the non-single crystalline film without causing much change in the temperature of the substrate.
The crystalline silicon film is formed from many crystal grains. Therefore, it is called a polycrystalline silicon film or a polycrystalline semiconductor film. A crystalline silicon film formed by laser annealing has high mobility. Accordingly, it is actively used in, for example, monolithic type liquid crystal electro-optical devices where thin film transistors (TFTs) are formed using this crystalline silicon film and used as TFTs for driving pixels and driving circuits formed on one glass substrate.
Furthermore, a method of performing laser annealing is one in which a laser beam emitted from a pulse oscillation type excimer laser, which is large in output, is processed by an optical system so that the laser beam thereof becomes a linear shape that is 10 cm long or greater or a square spot that is several cm square at an irradiated surface to thereby scan the laser beam (or relatively move the irradiation position of the laser beam to the irradiated surface). Because this method is high in productivity and industrially excellent, it is being preferably employed. A laser beam that has been linearized into a laser beam that is 10 cm long or greater at the irradiated surface is referred as a linear laser beam throughout the present specification.
Different from when using a spot shape (square) laser beam which requires a front, back, left, and right scan, when using the linear laser beam, in particular, the entire irradiated surface can be irradiated by the laser beam which requires only scanning at a right angle direction to the linear direction of the linear laser beam, resulting in the attainment of a high productivity. To scan in a direction at a right angle to the linear direction is the most effective scanning direction. Because a high productivity can be obtained, using the laser beam that is emitted from the pulse oscillation type excimer laser and processing it into a linear laser beam by an appropriate optical system for laser annealing at present is becoming a mainstream.
Shown in FIGS. 1A and 1B is an example of the structure of an optical system for linearizing the sectional shape of a laser beam on the irradiated surface. This structure is a very general one and all aforementioned optical systems conform to the structure of the optical system shown in FIGS. 1A to 1B. This structure of the optical system not only transforms the sectional shape of the laser beam into a linear shape, but also homogenizes the energy of the laser beam in the irradiated surface at the same time. Generally, an optical system that homogenizes the energy of a beam is referred to as a beam homogenizer.
If the excimer laser, which is ultraviolet light, is used as the light source, then the core of the above-mentioned optical system may be made of, for example, entirely quartz. The reason for using quartz resides in that a high transmittance can be obtained. Further, it is appropriate to use a coating in which a 99% or more transmittance can be obtained with respect to a wavelength of the excimer laser that is used.
The side view of FIG. 1A will be explained first. Laser beam emitted from a laser oscillator 101 is split at a right angle direction to the advancing direction of the laser beam by cylindrical lens arrays 102a and 102b. The right angle direction is referred to as a longitudinal direction throughout the present specification. When a mirror is placed along the optical system, the laser beams in the longitudinal direction will curve in the direction of light curved by the mirror. These laser beams in this structure are split into 4 beams. The split laser beams are then converged into 1 beam by a cylindrical lens 104. Then, the converged laser beam are split again and reflected at a mirror 107. Thereafter, the split laser beams are again converged into 1 beam at an irradiated surface 109 by a doublet cylindrical lens 108. A doublet cylindrical lens is a lens that is constructed of 2 pieces of cylindrical lenses. Thus, the energy in the width direction of the linear laser beam is homogenized and the length of the width direction of the linear laser beam is also determined.
The top view of FIG. 1B will be explained next. Laser beam emitted from the laser oscillator 101 is split at a right angle direction to the advancing direction of the laser beam and at a right angle direction to the longitudinal direction by a cylindrical lens array 103. The right angle direction is called a vertical direction throughout the present specification. When a mirror is placed along the optical system, the laser beams in the vertical direction will curve in the direction of light curved by the mirror. The laser beams in this structure is split into 7 beams. Thereafter, the split laser beams are converged into 1 beam at the irradiated surface 109 by the cylindrical lens 104. Thus, homogenization of the energy in the longitudinal direction of the linear laser beam is made and the length of the longitudinal direction is also determined.
The above lenses in the optical system are made of synthetic quartz for correspondence to excimer laser. Furthermore, coating is implemented on the surfaces of the lenses so that the excimer laser will be well transmitted. Therefore, the transmittance of excimer laser through one lens is 99% or more.
By irradiating the linear laser beam linearized by the above structure of the optical system in an overlapping manner with a gradual shift in the width direction thereof, laser annealing is implemented to the entire surface of a non-single crystal silicon film to thereby crystallize the non-single crystal silicon film and thus its crystallinity can be enhanced.
A typical method of manufacturing a semiconductor film that is to become the object to be irradiated is shown next.
First, a 5 inch square Corning 1737 substrate having a thickness of 0.7 mm is prepared as the substrate. Then a 200 nm-thick SiO2 film (silicon oxide film) is formed on the substrate and a 50 nm-thick amorphous silicon film (hereinafter denoted by “a-Si film”) is formed on the surface of the SiO2 film. Both films are formed by employing the plasma CVD apparatus.
The substrate is exposed under an atmosphere containing nitrogen gas at a temperature of 500° C. for 1 hour to thereby reduce the hydrogen concentration in the film. Accordingly, the laser resistance in the film is remarkably improved.
The XeCl excimer laser L3308 (wavelength: 308 nm, pulse width: 30 ns) manufactured by Lambda Co. is used as the laser apparatus. This laser apparatus generates a pulse oscillation laser and has the ability to output an energy of 500 mJ/pulse. The size of the laser beam at the exit of the laser beam is 10×30 mm (both half-width). Throughout the present specification, the exit of the laser beam is defined as the perpendicular plane in the advancing direction of the laser beam immediately after the laser beam is emitted from the laser irradiation apparatus.
The shape of the laser beam generated by the excimer laser is generally rectangular and is expressed by an aspect ratio which falls under the range of the order of 2 to 5. The intensity of the laser beam grows stronger towards the center of the beam and indicates the Gaussian distribution. The size of the laser beam processed by the optical system having the structure shown in FIG. 1 is transformed into a 125 mm×0.4 mm linear laser beam having a uniform energy distribution.
Based upon an experiment conducted by the present inventor, when irradiating a laser to the above-mentioned semiconductor film, the most suitable overlapping pitch is approximately 1/10 of the width (half-width) of the linear laser beam. The uniformity of the crystallinity in the film is thus improved. In the above example, the half-width of the linear laser beam was 0.4 mm, and therefore the pulse frequency of the excimer laser was set to 30 hertz and the scanning speed was set to 1.0 mm/s to thereby irradiate the laser beam. At this point, the energy density in the irradiated surface of the laser beam was set to 420 mJ/cm2. The method described so far is a very general method employed for crystallizing a semiconductor film by using a linear laser beam.
When an extremely attentive observation is made to a silicon film that has been laser annealed by using the above-mentioned linear laser beam, very faint interference patterns were seen in the film. The cause of the interference patterns seen in the film resides in that the laser beam is split and assembled in 1 region, and therefore the split light brings about interference with each other. However, because the coherent length of the excimer laser is about several microns to several tenths of microns, a strong interference will not occur. As a result, the influence imparted by the above-mentioned interference to a semiconductor device is extremely small.
The excimer laser is large in output and capable of oscillating pulses repetitively at a high frequency (approximately 300 hertz under the present situation), and hence is often used in performing crystallization of a semiconductor film. In recent years, advances have been made in manufacturing, low temperature poly-silicon TFTs used in liquid crystal displays. Accordingly, the excimer laser is employed in the crystallization process of semiconductor films.
Further, the largest output of an YAG laser is remarkably improved. Because the YAG laser is a solid state laser, it is easier to handle and maintain compared with the excimer laser which is a gas laser. Therefore, in the crystallization process of a semiconductor film, if the YAG laser is substituted for the excimer laser, an astounding improvement in cost performance can be expected. On the basis of the background such as the above, the present applicant is making an-examination in the possibility of using the YAG laser in the crystallization process of a semiconductor film.
It is known that the YAG laser outputs a laser beam having a wavelength of 1065 nm as the fundamental wave. The absorption coefficient of this laser beam with respect to silicon films is extremely low, and therefore the laser beam as it is cannot be used in the crystallization process of the a-Si film, which is one of the silicon films. However, the laser beam, i.e., the fundamental wave, can be modulated into having a shorter wavelength by using a non-linear optical crystal. Due to the modulated wavelengths, the laser beam is named a second harmonic (wavelength 533 nm), a third harmonic (wavelength 355 nm), a fourth harmonic (wavelength 266 nm), and a fifth harmonic (wavelength 213 nm).
Since the wavelength of the second harmonic is 533 nm, it has sufficient absorption to an a-Si film that is about 50 nm thick, and hence can be used in crystallizing the a-Si film. In addition, the third harmonic, the fourth harmonic, and the fifth harmonic also have a high absorption to the above-mentioned a-Si film, and therefore similar crystallization can be performed.
The largest output of the second harmonic from the current general-purpose YAG laser is about 1500 mJ/pulse. Further, the largest output of the third harmonic thereof is about 750 mJ/pulse and the largest output of the fourth harmonic thereof is about 200 mJ/pulse. The largest output of the fifth harmonic is further lower than the aforementioned largest outputs, and thus if the fifth harmonic is used in crystallizing the a-Si film, mass production will become extremely worse. Taking into consideration both the output of the laser beam and its absorption to the a-Si film, at the present level, it is best to use the second harmonic and the third harmonic.
In the case of using the YAG laser to crystallize the semiconductor film, nonetheless, the shape of the laser beam at the irradiated surface is preferably linear for mass production. Hereinbelow, an examination is made on the possibility of applying the above-mentioned optical system to the YAG laser without any modifications made thereto.
First, the difference between the beam shape of the YAG laser and the beam shape of the excimer laser will be described. The shape of the laser beam emitted from the excimer laser is generally rectangular, whereas the shape of the laser beam emitted from the YAG laser is both circular and rectangular. To process a rectangular laser beam into a linear laser beam is comparatively easy since transformation is made from a rectangular shape to a rectangular shape. However, to transform a circular beam into a rectangular linear laser beam is comparatively difficult. Therefore, judging from the shape of the beam only, it is better to use the rectangular beam emitted from the YAG laser than to use the circular beam emitted therefrom.
Hereinafter, a description will be made on the uniformity of energy between the beam of the YAG laser which emits the rectangular beam and the beam of the YAG laser which emits the circular beam, and an examination is conducted to discern which one of the YAG lasers is suitable as a substitute for the excimer laser.
The type of YAG laser which has a beam shape that is circular irradiates a strong light (flash lamp and laser diode, hereinafter denoted by LD) for exciting a cylindrical crystal rod, thereby obtaining laser oscillation. On the other hand, the type of YAG laser which has a beam shape that is rectangular irradiates a strong light to a parallelepiped crystal rod that structures a system called a zigzag slab, thereby obtaining laser oscillation.
Comparing the energy uniformity of the beam oscillated from the YAG laser which has a beam shape that is circular with that of the excimer laser, the energy uniformity of the former is in general not good. This non-uniformity of energy originates from a temperature distribution of the above-mentioned cylindrical crystal rod which occurs from the application of a strong light. As can be readily surmised from the shape of the aforementioned cylindrical crystal rode, the temperature of the temperature distribution thereof becomes lower towards the exterior of the cylinder. Thus, a function similar to that of a lens is added to the aforementioned cylindrical crystal rod, thereby worsening the energy uniformity of the beam. This phenomenon is generally called the thermal lens effect.
A system conceived for the purpose of restraining the above thermal lens effect is a zigzag slab system of the YAG laser. The structure of the zigzag slab system of the YAG laser will be briefly explained with reference to FIG. 2 in the following.
The rod system YAG laser obtains laser oscillation by excitation of the cylindrical crystal called a crystal rod. However, in the case of the zigzag slab system YAG laser, the shape of the crystal rod is parallelepiped. A parallelepiped crystal 202 is irradiated by excitation lamps 203a and 203b, for example, an LD and a flash lamp, to thereby obtain laser oscillation. Electric power is supplied to the excitation lamps 203a and 203b from a power source 208. Furthermore, the parallelepiped crystal 202 is cooled by a cooler 207.
Arranging resonant mirrors 201 and 204 diagonally to the parallelepiped crystal 202 is a characteristic of the zigzag slab system. The resonant mirrors 201 and 204 are arranged parallel in a state facing each other and sandwiching the parallelepiped crystal 202. Each of the surfaces of the parallelepiped crystal 202 and the resonant mirrors has no parallel positional relationship. By appropriately adjusting the positional relationship, light reflected from the resonant mirrors will advance in a zigzag way within the parallelepiped crystal. When laser is oscillated at this point in this state, a large amount of light will exit from a side surface of the parallelepiped crystal resulting in a large lost of energy, and thus becoming unusable. In order to prevent this drawback, reflector mirrors 205 and 206 are arranged at the side surfaces of the parallelepiped crystal to thereby prevent light escaping from the parallelepiped crystal 202. Gold-plated mirrors, for example, may be used as the reflector mirrors 205 and 206.
By adopting the above structure, the laser beam not only passes through the interior portion of the parallelepiped crystal rod, but also passes through the exterior portion thereof. Thus, influence to the biased laser beam of the temperature distribution of the crystal is less than the case of using the cylindrical crystal rod. Consequently, influence from the thermal lens effect becomes lesser thereby enhancing the uniformity of the beam.
Thus, it can be determined from the above examination that the zigzag system YAG laser is suitable as a substitute for the excimer laser than the type of YAG laser which employs the cylindrical crystal rod because the shape of the laser beam emitted from the zigzag system YAG laser is similar to that of the excimer laser and the uniformity of the beam thereof is higher.
Next, consideration is made in regards to a difference in the coherent length of the YAG laser and the excimer laser. As stated above, the coherent length of the excimer laser is about several microns to several tenths of microns, and therefore the occurrence of light interference when the laser beam passes through the aforementioned optical system, which splits and then converges the laser beam into 1 beam again, is thus very weak. On the other hand, the coherent length of the YAG laser is quite long, about 1 cm or more. Hence, the influences of the interference due to this long coherent length of the YAG laser cannot be ignored.
If the laser beam emitted from the YAG laser is passed through the optical system shown in FIG. 1 to be processed into a linear laser beam, then a linear laser beam 300 having an energy distribution with repetitive strong and weak regions in a lattice pattern as shown in FIG. 3A is formed.
The lattice pattern energy distribution is caused by the light interference. In FIG. 3A, darker lines 301 denotes regions where the energy is comparatively high and blank lines 302 between the darker lines 301 denotes regions where the energy is comparatively low.
Using the linear laser beam 300, which has a lattice pattern energy distribution, to crystallize the a-Si film will nonetheless cause non-uniform crystallization in the surface of the a-Si film. Shown in FIG. 3B is the appearance of a front surface of a silicon film 303 crystallized by the linear laser beam. As described in the above, the linear laser beam is irradiated on the a-Si film in the width direction of the linear laser beam and overlaps each other in a manner that is about 1/10 of the length of the width of the laser beam. Therefore, the stripes parallel to the linear direction of the linear laser beam eliminate each other out, becoming inconspicuous. However, lines 304 and 305 that are parallel to the width direction of the laser beam strongly remain. In FIG. 3B, darker lines 304 denotes regions where the energy is comparatively high and blank lines 305 between the darker lines 304 denotes regions where the energy is comparatively low.